Interior permanent magnet (IPM) machines are favored for fuel cell and hybrid electric vehicle operations due to their desirable characteristics, e.g., good torque density, high overall efficiency, and relatively constant power range, etc. The rotor field in a permanent magnet machine is obtained by virtue of its structure; unlike other machines such as induction, switched or synchronous reluctance machines, in which the field is generated by a stator current supplied by a source. As a result, permanent magnet machines exhibit superior efficiency as compared to other such machines.
An IPM machine may use either ferrite magnets or rare earth magnets, such as NdFeB. However, Ferrite magnets are often at risk for demagnetization under certain operating conditions (e.g., low temperature and high d-axis current). In order to allow the use of less expensive ferrite magnets, some IPM machines may include one or more rotor barriers, which include permanent magnets and/or air gaps (which may be filled with a nonmagnetic material). These rotor layers act as barriers to the permanent magnet field and lower a d-axis or the magnet maxis inductance thereby improving motor saliency. An increase in motor saliency improves motor efficiency and torque performance.